The invention relates generally to optical switching systems and more particularly to methods and apparatus for providing temperature stability to mirrors for reducing alignment variations during operation of the mirror due to environmental temperature changes.
In recent years optical fibers have come into wide spread use in a wide variety of applications in which optical signals are transmitted along such fibers and are switched from one fiber to another by means of an optical switch. Conventional optical switches generally include structure to support fiber positioning, alignment signal emitters and interconnected computer control electronics. A fiber positioning structure is provided near the end of each fiber to selectively point the end of a fiber in one fiber group toward the end of a selected fiber in another fiber group to provide switched optical transmission between the two fibers. An alignment signal emitter is provided near an end of and in predetermined spaced relationship to the end of each fiber to emit an alignment signal for controlling the fiber positioning structure when aligning the ends of selected fibers in the fiber groups for switched optical transmission there between. Examples are shown in U.S. Pat. Nos. 4,512,036 and 5,177,348. This approach requires considerable complexity and duplication of alignment structure and circuits for each alignable fiber. It would be advantageous to reduce this complexity and duplication and to increase speed of switching, reliability, as well as to reduce cost in implementation. As will also be appreciate by those skilled in the art, once alignment of the two fibers is complete, a large subsequent change in the operation temperature from that existing at the time of alignment may cause sufficient misalignment to interrupt light signal transfer between the two fibers. It would also be advantageous to reduce the effect of environmental temperature changes on the alignment and transmission of the fibers.
Texas Instruments presently manufactures a two-axis analog mirror MEMS device fabricated out of a single piece of material (such as silicon, for example) typically having a thickness of about 115 m. The layout consists of an oval mirror (normally about 3.8 mmxc3x973.2 mm) supported on a gimbal frame by two silicon torsional hinges. The gimbal frame is attached to a support frame by another set of torsional hinges.
The present invention is particularly suitable for reducing the effects of environmental temperature change on the two-axes analog mirror optical switch manufactured by Texas Instruments of Dallas, Tex. that overcomes the limitations of the prior art, and which is relatively low in cost and is reliable in operation.
For example, presently available optical transmission switches available from Texas Instruments employ a microelectromechanical (hereinafter MEM) movable mirror assembly with associated drive means such as for example only electromagnet coils, and may also include an LED and position control photo diodes with both drive and position control signals supplied through a standard connector or wiring harness. The drive signals to the electromagnetic coils, and signals to and from the positional electronics presently require a nine or ten wire connector and/or harness.
The mirror is typically mounted to a support structure of suitable material, such as ceramic, along with the driving means and a wiring harness. The mirror is mounted in a housing in which light from an optical fiber is received such that the mirror is disposed in alignment with the fiber for reflecting an incoming optical signal from the optical fiber to another optical fiber.
Objects and advantages of the invention will in part be obvious, and will in part appear hereinafter, and will be accomplished by the present invention which provides heat to the ceramic support structure and mirror to maintain the device within a selected temperature range. The apparatus and method of the invention is well suited for controlling the amount of heat to the combination mirror and support structure to accurately maintain the selected temperature range. The method and apparatus of the invention comprises a base support member which may be made of a ceramic or any other suitable material. A mirror rotatable about at least one set of axis is mounted on the support member. The mirror will operate over a wide range of temperatures, such as for example between 0xc2x0 C. and 70xc2x0 C. However, to assure proper alignment during operation, the present invention maintains the temperature range of the apparatus during operation over a much smaller range which includes the upper temperature limit of the allowable operating range. At least one PTC resistor having a switching temperature selected to be within about 20xc2x0 C. of the upper temperature limit is mounted on the support member so as to heat the mirror and support member up to the switching temperature of the PTC resistor. This maintains the temperature of the mirror between a temperature range of the switching temperature of the PTC resistor and the upper temperature limit of the allowable operating range of the mirror. The mirror is operated over this reduced temperature range and thus any misalignment due to temperature variation is substantially reduced.
There is also included at least one drive module such as for example only an electromagnetic coil or alternately an electrostatic plate located on the topside of the base support board and which has input connections. The drive module is used for providing rotational forces to the optical mirror mounted on the topside of the support board and above the drive module. If the drive module is a single coil, the coil may be used to cause rotation of the mirror about an axis in a first direction by providing current flow in one direction. Similarly, rotation may be provided in the other direction by reversing the current flow. The rotation around the axis, however, may also be more readily accomplished by using two drive coils instead of a single coil. Likewise, as is discussed in detail hereinafter, rotation of the mirror about two axes may be accomplished by using two coils (one coil per axis) or four coils (a pair of coils per axis) or alternately, by using electrostatic plates.
The mirror used in the assembly, (both one axis of rotation or two axis of rotation) is preferably made from a single piece of crystalline material such as silicon and has three portions connected by two sets of hinges. An inner portion forms the mirror and is hinged on each of two opposite sides of the mirror portion, to a middle gimbals portion, which surrounds the mirror portion. This allows the mirror portion to rotate about the gimbals portion, providing the first axis of rotation. A second set of hinges attaches the gimbals portion to a frame portion by a pair of hinges having one hinge on each of two opposite sides on a line disposed, preferably orthogonal or 90xc2x0 relative to a line drawn through the first set of hinges. This allows the gimbals portion, which carries the mirror, to rotate about the frame portion, providing a second axis of rotation.
In one embodiment, two pair of magnets, one pair for each axis of rotation, are used to increase the magnetic response of the mirror portion and are mounted to the mirror portion and the gimbals portion. The first pair of magnets are attached by suitable means to the mirror portion of the mirror assembly, one on each of two opposite sides of a line, 90xc2x0 relative to a line through the mirror/gimbals portions set of hinges. When subjected to a magnetic field, the mirror portion rotates about the mirror/gimbals portions set of hinges, providing the first axis of motion. The second pair of magnets are suitably attached to the gimbals portion of the mirror assembly, one on each of two opposite sides of a line, 90xc2x0 relative to a line drawn through the gimbals/frame portions set of hinges. In the same manner as discussed above, when subjected to a magnetic field, the mirror and gimbals portions rotate about the second set of axis, to providing the second axis of rotation.
To obtain extended operation without degradation, the mirror assembly may be hermetically assembled into a cavity in the package to lock out moisture and allow the provision of a benign atmosphere for mirror operation. The cavity can be filled with selected gases to provide improved heat transfer and, if desired, exclude oxygen water vapor and other materials that would adversely affect the mirror over time.
According to one embodiment using drive coils as the drive modules, the drive coils preferably employ a push and pull arrangement for driving the mirror magnets to rotate the mirror portion to the desired orientation in its two axes. The coil leads are connected to control electronics to allow system electrical control of the coils and their push pull arrangement to drive the mirror assembly. The drive coils are wound on bobbins preferably made of aluminum or other eddy current generating material, and sufficient amounts of aluminum should be provided at the top and bottom of the bobbins to allow eddy current dampening of the movable portions of the mirror assembly, to prevent unwanted oscillations.